Thermally insulating panels are typically constructed by depositing a strip of solder glass around the periphery of the glass sheets, depositing an array of support pillars onto one or other of the sheets of glass, bringing the sheets together or permitting them to move together, heating the panels to melt the solder glass around the periphery so that the two sheets settle onto the pillars, and then cooling the panel to solidify the edge seal. The panel is the evacuated by pumping out through a tube positioned either through one of the glass sheets or through the edge seal, and finally the pump-out tube is melted and sealed off.
The improvement concerns the edge seal, and in particular a method of constructing a thermally insulating glass panel with a superior edge seal, and a panel incorporating the superior seal.
Several conditions must be satisfied in the construction of evacuated glazing. First, it is necessary to produce an hermetic (leak-free) edge seal around the periphery of the panel. Second, in order to prevent the glass sheets from being forced into contact with each other, by the large atmospheric pressure forces acting, it is essential to include an array of support pillars within the glazing. Third, the space between the glass sheets must be evacuated to a high level, and this level of vacuum maintained over the life of the glazing. Fourth, in order to reduce radiative heat flow through the glass, a low emittance coating is applied to the inner surfaces of one or both sheets of glass.
One of the methods of forming the edge seal around the periphery of evacuated glazing is to use a low melting point glass (solder glass). In order to manufacture such an edge seal, solder glass is deposited as a liquid slurry onto the surface of one or both of the glass sheets, around the edges. The glass sheets are brought together and the entire assembly is heated to a temperature at which the solder glass melts, wets the surface of the glass sheets, and flows to produce an hermetic seal between the sheets.
One of the difficulties is that even at the high temperatures used the solder glass possesses a relatively high viscosity, and it does not flow readily between the glass sheets. If there is too much solder glass in the gap between the sheets, it may not flow out of the gaps sufficiently to allow the upper sheet to settle down onto the pillars during the edge sealing process. This leads to large tensile stresses in the glass sheets near their edges when the vacuum is eventually created.